Multiple locations and clustering

Hi, I am designing solution for massive amount of users for presence and messaging.
I would like to create several pops for ejabberd servers and manage users using geo-ip load balancer.
In addition I would like to keep cluster for each location.

Questions:
1. Is this sounds reasonable?
2. Regarding DB, what is the preferred DB (LDAP/mysql/mnesia) and is it better to have it distributed or centralized?
3. Is it better to use all nodes with the same global domain, or use different subdomains? is it possible to use the same subdomain not on a different location?

I am familiar with ejabberd and erlang programming, add-ons etc.

Thanks,
Gadi

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